The present invention relates to a radio receiving apparatus that can suppress the output of detected signals in accordance with the amount of noise components.
In a known FM (Frequency Modulation) radio receiving apparatus, an IF (Intermediate Frequency) signal and then a baseband signal are extracted from an RF (Radio Frequency) signal received at an antenna. The baseband signal is supplied to a detector to be a demodulated signal that is then output through a speaker. Not only the demodulated signal, noise is given off from the speaker, based on a unwanted low-level radio wave when an RF signal to be processed is not received.
To solve such a problem of noises, in another known FM radio receiving apparatus, noise components are only extracted from a demodulated signal, that are then rectified and smoothed to produce a squelch signal. The output of the demodulated signal to a speaker is cut whenever the voltage level of the squelch signal reaches a predetermined threshold level (squelch control.) A demodulated signal is output to the speaker whenever an RF signal to be processed is received under the squelch control.
The latter known FM radio receiving apparatus is configured with analog circuitry. A received signal thus can be amplified to be distorted before the detection by a discriminator. There is also small leakage noise from an analog filter even in bands other than the passband. It is thus easy to detect noise components from a demodulated signal, so that the squelch control is successfully performed.
Recent FM radio receiving apparatuses are, however, configured with digital circuitry in order to meet the demands for narrower occupied bandwidth, higher speech quality, and higher speech secrecy. In such digital FM radio receiving apparatuses, the following are advanced: narrower occupied bandwidth in analog frequency modulation; and further narrower occupied bandwidth by digital modulation with 4-level FSK (Frequency Shift Keying.) In detection, in place of a discriminator suitable for analog modulation, the arctangent function is employed that applies inverse orthogonal transform to the value obtained by adding angular displacement corresponding to a modulated signal to the former value, through the tangent function.
With the arctangent function, however, a received signal cannot be saturated due to digital processing with A/D conversion. Thus, with the arctangent function, it is impossible to perform detection, such as a discriminator, to a signal saturated by a limiter to have a limited amplitude. Therefore, it is difficult to obtain noise enough for squelch control, with the arctangent function.
Moreover, narrower occupied bandwidth makes it difficult for an analog filter to achieve steep characteristics for adjacent channel rejection. Adjacent channel rejection (referred to as ACR, hereinafter) in bands other than the passband can be achieved with a digital filter that exhibits the characteristics with a steep attenuation curve.
However, an ACR filter, for example, inevitably suppresses leakage noise that would otherwise be detected due to poor ACR performance of an analog filter if used and also cannot saturate a received signal with a limiter. With the ACR filter, it is thus difficult to detect noise components of a demodulated signal, resulting in poor performance of squelch control. Thus, users are forced to listen to unwanted noise.
Accordingly, in known radio receiving apparatuses installed with a digital modulation mode, digital saturation processing is performed to very small noise components demodulated by a detector. In the digital saturation processing, for example, a noise level is replaced with a predetermined upper limit value if it reaches a predetermined threshold level, as if the noise components were amplified.
Such digital saturation processing allows the detection of noise components, however, irregular harmonics inevitably appear because the processing is nonlinear. Squelch control functions normally if the level of the irregular harmonics is lower than a predetermined threshold level that is used for determination in squelch control. However, squelch control functions more than necessary if the level of the irregular harmonics reaches the threshold level.
It is thus desirable for squelch control to detect noise components while avoiding irregular harmonics. However, it is impossible to set a frequency band for detecting noise components if the frequency at which irregular harmonics appear is not constant. The frequency band for noise detection is limited to a narrow band, even if it can be set, which can avoid the appearance of irregular harmonics.
Thus, when irregular harmonics appear in a frequency band for noise detection, with the levels reaching the threshold level, discussed above, the irregular harmonics are detected as a squelch voltage used for squelch control. The squelch control thus inevitably functions to block the output of a demodulated signal that should not be blocked.